1. Field of Invention
The present invention relates to a drive mechanism for a pump, more specifically to a variable drive mechanism for a hydraulic pump.
2. Description of Related Art
From the prior art variable displacement pumps are known, wherein the displacement volume and the volume flow are mechanically adjustable. Vane-cell pumps and piston pumps are suited for adjustment in this way. These pumps only deliver a required amount of fluid, with adjustment being effected within the pump. It is, however, a disadvantageous effect that movable parts must be adjusted mechanically whereby accuracy is impaired, that orifices reducing the efficiency are used for volume flow control, and that the rotational speed is limited more narrowly than in the case of fixed displacement pumps.
In order to remedy these problems, polyphase machines are employed as a drive mechanism for a pump in the prior art, the input voltage and input frequency of which are variable by means of a frequency converter. The frequency converter may be physically separate from the polyphase machine or coupled to it by screw connection. Direct AC converters may be employed as frequency converters, whereby the motor winding is switched to different outer conductor voltages. It is, however, a drawback that the output frequency may only be adjusted in large steps, and that only frequencies below the mains frequency are possible.
These drawbacks are are eliminated in a frequency converter including an intermediary circuit. In such a link converter, the mains voltage is rectified to a direct voltage, smoothed, and applied to a DC-AC converter. At the output of the DC-AC converter, an alternating current having a different voltage and a different frequency is then available, which will then arrive in the polyphase machine. Polyphase machines preferred for economic reasons are the widely spread three-phase asynchronous machines. Such a motor features availability of a multiplicity of types and the fact that it does not require any startup aids.
Due to eddy currents and magnetic reversals in the magnetic material and due to the effect of the electrical current in the coil resistances of the above mentioned polyphase machines, heat is produced which limits the useful performance. In order to dissipate this heat, surface cooling is typically employed in the three-phase asynchronous machine, whereby ventilator losses are additionally incurred.
A fan-type air cooling which reduces the temperature at the surface is shown in FIG. 1. Herein a rotor 102 and the stator coil 101 on which heat is produced are located inside a housing 106. On the housing 106, radiator fins 105 are provided which expose the generated heat to air on a large surface. A fan 103 increases the flow velocity of the intake air and guides it past protection caps 104 across the radiator fins 105 where the air may adsorb the generated heat.
Inasmuch as the cooling medium used is a gas and is sufficiently available in the environment, surface cooling is an economically favorable manner of proceeding if cooling may take place independently of engine speed, the amount of heat to be dissipated does not exceed a certain limit, and air cleanness and humidity suffice specific minimum requirements.
At the frequency converter, too, energy is generated which cannot be transferred to the pump. Generatory energy is re-supplied into the mains in the case of very large drive mechanisms upwards of approximately 30 kW. In the case of drive mechanisms in the medium or lower performance range, a braking resistance is inserted in the intermediary circuit whenever the intermediary circuit voltage exceeds a particular value. At this braking resistance, the conversion of excessive energy into heat, which is transferred from the housing to the surrounding air, takes place.
It is the object of the present invention to optimise and efficiently execute cooling of a drive mechanism for a pump including a drive motor, wherein high compactness of the device is to be ensured.
Moreover in the present invention, the efficiency of a standard asynchronous motor is to be enhanced, and a large quantity of heat is to be dissipated from the standard asynchronous motor.
It is furthermore desired to reduce the airborne noise emission of a combination of drive motor and pump.
This object is attained through a drive mechanism for a pump including a drive motor in accordance with claim 1. Herein a frequency converter for actuation of a drive motor is mounted on a heat dissipator cooled by the hydraulic fluid to be conveyed by the pump. Thus, in the absence of an additional cooling medium, the temperature of the frequency converter may be reduced and its operation be designed to be more effective.
In a preferred manner, the heat dissipator is located at the suction port of the pump in order for the pressure and the volume flow at the pressure port of the pump to be at the desired values.
Even though more intense cooling of the drive motor might be desired or required, it is favorable to arrange this drive motor inside the heat dissipator in such a way that maximum enveloping flow takes place. In this way, the heat transfer from the drive motor to the pressure liquid thus also used as a cooling liquid may be optimised.
For the heat dissipator any tubular shape is well suited inasmuch as in this way the number of dead zones is minimised, and at the same time a uniform enveloping flow around the drive motor may take place in the heat dissipator. Moreover it is thereby also possible to favorably solve problems with respect to sealing of the heat dissipator.
When using a standard asynchronous motor, the A and B flanges may be fastened on both sides of the tubular main portion of the heat dissipator, so that the drive mechanism according to the present invention may be used on already existing drive motors in a simple manner. Preferably seals are introduced between between the cyclindrical portion of the motor and the respective flanges in order to isolate the inside of the motor from the cavity of the heat dissipator.
When the electrical connection between the drive motor and the frequency converter is routed through a flange lid, an impairment of the cable for electrical actuation of the drive motor due to the hydraulic flow in the heat dissipator is avoided.
A fan may be provided on a drive motor fan shaft externally of the heat dissipator in order to cause movement of air around the heat dissipator. Hereby the temperature of the pressure liquid supplied to the pump is reduced.
The port leading to the tank and the port leading to the pump are preferably located outside the mechanical connection between the flanges and the flange lids. Due to such an arrangement, mechanically stable installation of the drive motor into the heat dissipator is made possible, and a rapid movement of pressure liquid through the heat dissipator is equally permitted.
In order to avoid rectilinear, preferred flow paths in the heat dissipator, the ports in another embodiment of the present invention are offset with respect to the output shaft of the drive motor, i.e., they are formed in different relative positions on the flange lids. Uniform flow through all ranges of the heat dissipator is hereby achieved.
High compactness of the drive mechanism according to the present invention is ensured in the present invention through the fact that the front face-side lid of the drive motor is concurrently used as the sealing cover for the pump.
Preferably a fluid conduit connecting the space adjacent the drive motor with the suction port of the pump extends in the front face-side lid. Hereby an arrangement of conduits externally of the drive mechanism is avoided, and operational safety is enhanced.
In a preferred embodiment, the space adjacent the drive motor is the cavity in the heat dissipator, whereby a space-saving arrangement of pump, drive motor and frequency converter at concurrent efficient cooling of motor and frequency converter is made possible.
It is furthermore an advantage that the pump is an axial piston pump, and support means are provided at the front face-side lid as an abutment for the axial pistons. As a result of these support means, vibrations of the pump may be minimised because a design of the support means becomes possible which is conceived with a view to the mechanical properties only.
Radially outside of the support means, a housing comprised of sound-insulating material and fastened to the front face-side lid may be provided. As this housing is mechanically connected at only one end portion with the support means, preferably insulated against structure-borne noise, the acoustic emission of the pump is reduced.
In another preferred embodiment, the opposite end portions of the drive motor are secured to the heat dissipator through flange lids of flexible material. Such a structure further contributes to increased sound attenuation of the drive mechanism.
Further developments in accordance with the invention constitute the subject matters of the dependent claims.